1. Field of the Invention
The present invention relates to surface emitting lasers and photodetectors for use in optical communication with polarization division multiplexing and optical communication systems using the lasers and the photodetectors.
2. Description of the Related Art
To meet with recent increase of an information communication demand, increases of communication speed and communication capacity are urgently needed. Accordingly, modulation bandwidths of elements for mutually converting electrical signals and optical signals in, for example, a laser in a transmitting unit and a photodetector in a receiving unit are increased and transmission capacity is increased by adopting a communication scheme such as wavelength multiplexing or polarization division multiplexing. The polarization division multiplexing with which optical signals in different polarization states are transmitted at a time using one optical fiber allows the number of channels to be increased to a number corresponding to the number of polarization states and enables increase of communication capacity.
As a means of generating a polarization multiplexed optical signal, the following example is disclosed in Japanese Unexamined Patent Publication No. 2003-124913. As illustrated in FIG. 19, outputs of two lasers are coupled to respective polarization-preserving optical fibers, and the two polarization-preserving optical fibers are connected to the input ends of a polarization multiplexing prism 201 such that polarization directions are orthogonal to each other. A signal transmitted to the optical fiber from the output end of the polarization multiplexing prism 201 becomes a polarization multiplexed optical signal including optical signals in two polarization states in which polarization directions are orthogonal to each other.
In the conventional configuration, however, the polarization multiplexing prism, two polarization-preserving optical fibers provided at the input ends and one polarization-preserving optical fiber provided on the output end are necessary as a means of generating a polarization multiplexed optical signal. This causes the problems of increase in the number of parts, increase in the size of a system and increase in cost.
In addition, a coupling unit for coupling the prism and the optical fibers is also needed, so that long-distance communication becomes difficult because of a coupling loss. Moreover, the two optical fibers need to be coupled to the prism such that the polarization directions are orthogonal to each other. Accordingly, a number of processes enough to adjust, for example, the positions of the optical fibers and the coupling angles between the optical fibers and the prism are needed, so that cost reduction is difficult.
In the case of receiving a polarization multiplexed optical signal in multiple channels, it is necessary to provide analyzers and photodetectors in a number corresponding to the number of the polarization states included in the polarization multiplexed optical signal, so that a receiving unit needs to be simplified. In addition, the polarization multiplexed signal needs to be divided using, for example, a prism, resulting in a decrease of signal intensity. As a result, the problem of decrease of the S/N ratio and other problems arise.